The present invention is a method of uniformly applying a foamed adhesive to a moving substrate which is subject to starts and stops at irregular intervals.
Foamed adhesives have been investigated for the preparation of various types of laminates such as plywood and corrugated board. The use of a foamed adhesive is said to convey a number of advantages. A principal claim has been reduced adhesive usage. One reason for this is believed to be decreased penetration of the adhesive into the substrate material. In plywood manufacture, claims have also been made for shorter press times when a foamed adhesive is used.
Foamed adhesives in themselves are not new. Their preparation and use can be exemplified in the following United States Patents to Cone and Steinberg; U.S. Nat. Nos. 3,895,984, 3,905,329, 3,905,921, 3,965,860 and 4,258,088. Their practicality for plywood manufacture came about with the advent of continuous lay-up lines about 1960. These lines are subject to frequent and unpredictable stops which may be from a few seconds to a half-hour or more in duration. It was not too difficult to engineer a system for making and applying a foamed glue which would work satisfactorily on a continuously running lay-up line. However, the stops and starts have presented a problem which has not been satisfactorily overcome until the time of the present invention. The major problem to be overcome is that of over application or under application of adhesive to the veneer between the time when a signal is given to stop the line until the line is once again operating at full speed.
The conveyor lines which carry the veneers are massive in size and have considerable inertia to overcome in a start-stop cycle. As a result, when a stop signal is given, the line may travel as much as two meters before it comes to a complete stop. During this time period the line speed is continuously decreasing. Therefore, if adhesive is applied at a constant rate per unit of time the veneer will be overspread during the decleration period. The usual result of overspreading in plywood manufacture is a poor bond because the thermosetting adhesive in the overspread area will be undercured. Frequently, a "blow" will result. This is a circumstance where a steam pocket forms between the veneers due to the excessive moisture in the overspread area. The result is a partially delaminated product which is either unsalable or must be seriously downgraded in value.
In an attempt to control overspreading, the results have often been at the opposite extreme. In this case, a portion of veneer is underspread or no adhesive at all is applied during all, or a portion, of the start-stop cycle. The ultimate result is the same and the product contains delaminations which render it generally unsalable.
Cone et al. in U.S. Pat. No. 3,905,329 describe a system in which foamed glue is applied to wood veneer through an extrusion head containing a plurality of orifices. When a sensor detects that the veneer has stopped its movement, an electrically operated valve redirects the foamed adhesive away from the extrusion head and into a defoaming device. The defoamed glue is recycled into a holding tank where it is mixed with fresh adhesive entering the tank. The system continues to operate with foamed glue being recycled until the sensor detects that the assembly line is once again moving. This patent does not indicate that any provision has been made for keeping glue spread uniform in the decleration period prior to a complete stop, and the acceleration period during a start-up.
Spread uniformity across a substrate of some width is another problem that has bothered foamed adhesive systems. Cone and Steinberg, in U.S. Pat. No. 3,895,984 show several manifold constructions which are designed to achieve uniform flow from each orifice in the manifold. They also show one construction in which adhesive flow to the substrate can be stopped through the action of a slide valve located immediately above the orifces or extrusion nozzles in the manifold. The sequence of events during a start-up or shut down are similar to those described in U.S. Pat. No. 3,905,329, except that a second valve, acting in concert with the slide valve, serves to divert the flow of foamed adhesive back through the defoamer.
Other inventors have taken a different track to the achievement of uniform flow to a plurality of nozzles mounted on a manifold. Winstead in U.S. Pat. No. 2,734,224 and Wells in U.S. Pat. No. 3,381,336 have employed a channel system which repeatedly bifurcates until each nozzle is individually fed from its own channel. As each larger channel divides into smaller ones, the cross-sectional area is reduced so that the sum of the area of the smaller channels is approximately equal to the area of the larger channel. Additionally, any given point downstream from a bifurcation is equidistant from the point at which the division occurs.
Foaming devices themselves are well known in the art. Examples might be those described in the patents to Jurgensen, Jr., U.S. Pat. No. 2,695,246 and Oakes, U.S. Pat. No. 3,081,069. The former device was designed specifically to be useful in the preparation to foamed rubbers while the latter is useful in a very wide variety of foamed products. It has found considerable use within the food industry. Unfortunately, no satisfactory system is found in the prior art to deal with the problem of uneven longitudinal adhesive application to a substrate which is subject to irregular starts and stops.